This invention concerns exciter apparatus for use in acoustic signature inspection of railroad car wheels.
Acoustic signature inspection provides an automated technique for detecting cracked wheels on moving railroad cars. This technique involves use of a trackside exciter which strikes a passing wheel to produce a sound pressure level which is significantly greater than ambient noise, and a spectrum analyzer which interprets the resulting sound spectrum of the impacted wheel. Data from the analyzer is read by a digital computer, which compares it with corresponding information obtained from the similarly excited opposite wheel on the same axle, and then makes a wheel evaluation using the assumption that the spectra of two good wheels on a common axle will be the same. Fuller discussions of the complete inspection process are contained in the following reports:
"Operational Parameters In Acoustic Signature Inspection of Railroad Wheels, D. Dousis and R. D. Finch, University of Houston, Final Report October, 1978 (prepared for U.S. Department of Transportation under Contract DOT-TSC-1187). PA1 "Test Report For The Cracked Plate Detector Tests (Series 1) At The Transportation Test Center, November, 1980 (prepared by University of Houston under Subcontract No. W-0276 to The Aerospace Corporation for U.S. Department of Transportation, Contract No. FO 4701-C-0078).
The exciter employed in an acoustic inspection system must be capable of producing the required sound pressure level in a train speed range of about 2-15 MPH, and the resulting sound signature must be reproducible and independent of train speed. In addition, the exciter must be suitable for use outdoors at virtually any location, present no hazard to the train, satisfy normal railroad requirements for trackside apparatus, including personnel safety requirements, and be sufficiently durable to have a reasonable useful life. As indicated in the reports mentioned earlier, various exciters prior to mine were considered and/or evaluated and found unacceptable.
The object of the present invention is to provide an improved form of exciter which satisfies fully the requirements of a practical acoustic signature inspection system. According to the invention, the new exciter takes the form of an electromagnetic hammer, the mechanical portion of which comprises a pivotally mounted impact member which is actuated by a solenoid through a special lost motion connection. That connection is designed so that the solenoid moves the impact member through only an initial phase of its impact stroke, thereby imparting to the member sufficient kinetic energy to carry it through the balance of that stroke and into contact with the passing wheel. The solenoid is controlled by an electronic drive circuit which responds to an input signal from a wheel presence detector and delivers to the solenoid coil a burst of electrical energy whose magnitude and duration are selected, and preferably adjustable, to yield the maximum sound pressure level from the impact. The circuit enables the magnetic field of the coil to relax quickly so that the apparatus will reset in the short time interval between passage of successive wheels, even at relatively high train speeds.
The magnitude of the impact produced by the new exciter is independent of train speed. Moreover, since the impact member is not coupled rigidly to the solenoid armature throughout the stroke, but has a range of free motion, the mechanical and electrical portions of the apparatus may be field tuned spearately for optimum performance. As a result, the apparatus can operate and produce a relatively uniform and sufficiently great sound pressure level at any train speed within the range of about 2-15 MPH.